Much of the transportation of present-day goods and raw materials is accomplished by "over-the-road" vehicles powered by internal combustion engines. The load may be carried in a truck body unitary with a power plant, or in a separate trailer drawn by a removable traction vehicle. It is conventional for the vehicle to include a cab for the driver which is unitary with the power plant: the tractor or truck body is taller than the cab and in order to maximize loading space has a square vertical front which offers very considerable wind resistance when the vehicle is in motion. Resort has been had to deflectors mounted on the cab roofs to modify the air flow so that the air stream passes above the front of the body or trailer instead of impinging directly on it.
It is also customary to conduct the products of combustion of the power plant to an exhaust pipe and muffler and discharge them at a height above the cab roof, and to draw air for the engine through an air cleaner from an inlet also elevated above the cab roof. Problems of sound generation and wind resistance are present, as is the need for special design to keep precipitation out of the inlet and exhaust system.